1. Field of the Invention
The present invention relates to a piezoelectric vibration component, specifically a piezoelectric vibration component which uses an energy trapped type thickness slip vibration mode. Furthermore, a piezoelectric vibration component according to the present invention can be used as a piezoelectric trap, a piezoelectric filter, a piezoelectric discriminator, or a piezoelectric oscillator.
2. Description of the Prior Art
FIG. 8 shows a piezoelectric trap element which is one conventional example of a member of a piezoelectric vibration component. Piezoelectric substrate 21 is formed into a rectangular shape from a ceramic or other piezoelectric material. The substrate is processed so that the direction of polarization axis P is parallel to the long side of the piezoelectric substrate 21. Furthermore, a vibrating electrode member 22 and a terminal member 23 are formed by vapor deposition, sputtering, or other thin film formation technique on the front and back surfaces of piezoelectric substrate 21. With this piezoelectric vibration element D, energy trap-type thickness slip vibrations are set up in piezoelectric substrate 21 when an alternating current signal is applied to vibrating electrode member 22. The resonance frequency fo of these vibrations is determined by the thickness of the piezoelectric substrate 21, the mass of the vibrating electrode member 22, and other factors.
Moreover, this piezoelectric vibration element D forms piezoelectric vibration member 26 (FIG. 9) when a lead terminal (not shown in the figures) is soldered to terminal member 23, and packaging resin layer 24 is formed around piezoelectric vibration element D by an epoxy resin or other packaging resin. Therefore, as shown in FIG. 9, piezoelectric substrate 21 is held firmly at a position inside packaging resin layer 24, and a hollow vibration space 25 is formed between packaging resin layer 24 and vibrating electrode member 22 so that the vibrations in vibrating electrode member 22 are not damped by packaging resin layer 24.
Because piezoelectric vibration element D is thus covered by packaging resin layer 24 to form piezoelectric vibration member 26, clamping stress from packaging resin layer 24 may be applied, and thermal stress between packaging resin layer 24 and piezoelectric substrate 21 may occur due to the difference in the coefficients of thermal expansion of packaging resin layer 24 and piezoelectric substrate 21, or to changes in the operating temperature. If a stress F is applied, due to one of these factors, in the clamping direction to piezoelectric substrate 21, a change (shift) from the design value will occur in resonance frequency fo as indicated by A in FIG. 4. Thus, the greater stress F becomes, the greater becomes the change .DELTA.-f in the resonance frequency. Possible reasons for this include the following. Specifically, due to contraction deformation in the length and width directions of piezoelectric substrate 21 caused by stress F, elongation deformation in the thickness direction of piezoelectric substrate 21 arises at vibrating electrode member 22 enclosed within vibration space 25. Thus, the frequency of the standing wave occurring in the thickness direction of piezoelectric substrate 21 in vibrating electrode member 22 decreases, and it may be concluded that as a result the frequency of the wave propagated in the lengthwise direction increases.
Therefore, in a conventional piezoelectric vibration component, when stress is applied to the piezoelectric substrate from the external resin layer, a large change proportional to the resonance frequency will occur which decreases the reliability of the piezoelectric vibration component. In particular, because a change in the resonance frequency occurs due to the thermal stress accompanying a temperature change, there have been problems in the thermal characteristics of piezoelectric vibration components. To resolve these problems in conventional components, research has been advanced in the development of new piezoelectric materials characterized by minimal change in the resonance frequency, and in the development of suitable packaging resins to achieve a minimal difference in the coefficients of thermal expansion between the piezoelectric substrate and the packaging material.
The present invention has as its objective to reduce the change in the resonance frequency produced in conjunction with temperature changes, and improve the characteristics of the piezoelectric vibration component.
To this end, we inventors prepared various piezoelectric vibration elements using a piezoelectric substrate processed with the polarity aligned in the lengthwise direction and in which the length of the long side and short side of the piezoelectric substrate was changed. A compression stress was applied to these various piezoelectric vibration element materials, and the change in the resonance frequency at that time was measured. As a result of these measurements, it was found that the rate of change of the resonance frequency to stress (the tangent of lines A and B in FIG. 4) increased proportionally to the length of the long side of the piezoelectric substrate. However, piezoelectric vibration elements have already been reduced as much as possible in size due to the demand for the micronization of components, and there is a limit to how much the length of the piezoelectric substrate can be reduced due to the dimensions of the vibration electrode member and other technological restrictions. Therefore, we further extended the scope of our experiments and manufactured a piezoelectric vibration element in which the polarization axis is parallel for all intents and purposes to the short side of the piezoelectric substrate (in a conventional piezoelectric vibration element, the direction of the polarization axis is parallel to the long side of the substrate). These samples were then used in tests in which the amount of compression stress applied to the piezoelectric substrate was varied and the resonance frequency was measured, and it was found that the change in the resonance frequency was small.
A comparison of the change in the resonance frequency of piezoelectric vibration elements of the same dimension when the polarization axis is parallel to the long side and when parallel to the short side is shown in the graph of FIG. 4. The x-axis is the stress applied in the clamping direction, and the y-axis shows the change in resonance frequency. Line A represents the piezoelectric vibration element in which the polarization axis is parallel to the long side of the piezoelectric substrate, and B represents the piezoelectric vibration element in which the polarization axis is parallel to the short side of the piezoelectric substrate. As is obvious from the graph, the change (and the rate of change) in the resonance frequency is significantly less in a piezoelectric vibration element with the polarization axis parallel to the short side of the piezoelectric substrate than in a piezoelectric vibration element in which the polarization axis is parallel to the long side of the piezoelectric substrate.
From the above experiments, we came to a conclusion that the change in the resonance frequency can be more efficiently reduced not by shortening the length of the long side of the piezoelectric substrate, but by actually reducing the dimension of the piezoelectric substrate in the direction of the polarization axis.